Updated 9/09

GeoMan's Glossary of Earth Science Terms




Aa: Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments. Aa flows commonly develop from pahoehoe flows as they cool and lose gas.

Absolute Date: An estimate of the true age of a mineral or rock based on the rate of decay of radioactive minerals.

Acre-foot: The volume of water required to cover one acre of land to a depth of one (1) foot. One acre-foot is equal to 325,851 gallons of water.

Active volcano: A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.

Adiabatic rate: The rate of temperature change in the atmosphere due to the raising or lowering of an air mass. The "dry adiabatic rate" is 5.5 deg. F. per 1000 feet, while the "wet" rate is 3.5 deg. F. per 1000 feet.

Alluvium: A general term for clay, silt, sand, gravel or similar unconsolidated material deposited by a stream or other body of running water.

Andesite: Intermediate volcanic rocks containing 54 to 62 percent silica and moderate amounts of iron and magnesium. Andesite minerals commonly include plagioclase and hornblende, with lesser amounts of mica, pyroxene, and various accessory minerals. Andesites are aphanitic in texture and are usually medium dark in color. They occur with composite volcanic cones associated with convergent plate margins.

Aquifer: A water-bearing layer of rock or sediment capable of holding and transmitting fluid (such as water, gas, or oil).

Aquifer, Confined (or Artesian): An aquifer overlain by a non-permeable layer or layers, in which pressure will force water to rise above the aquifer.

Aquifer, Perched: An aquifer containing unconfined groundwater separated from an underlying body of groundwater by an unsaturated zone.

Aquifer, Principal: The aquifer or combination of related aquifers in a given area that is the important economic source of water to wells.

Aquifer, Secondary: Any aquifer that is not the main source of water to wells in a given area.

Aquifer, Unconfined (or Water Table): An aquifer in which the upper surface is the water table.

Aquiclude: An impermeable geologic formation or stratum which will not hold or transmit fluid.

Aquitard: A geologic formation or stratum that significantly retards fluid movement.

Artesian Well: A well in an aquifer where the groundwater is confined under pressure and the water level will rise above the top of the confined aquifer.

Artificial Recharge: The unnatural addition of surface waters to groundwater. Recharge could result from reservoirs, storage basins, leaky canals, direct injection of water into an aquifer, or by spreading water over a large land surface.

Ash: Fine particles of rock material ejected during an explosive volcanic eruption (commonly intermediate to felsic events). Ash may be either solid or molten when first erupted, and generally measures less than 0.10 inch in size (larger particles have other names).

Ashfall (subaerial): Volcanic ash that has fallen through the air. The resulting deposit is usually well sorted and exhibits a finely layered structure.

Ash flow: A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure. The mass of pyroclastics is normally of very high temperature and moves rapidly down the slopes, or even along a level surface.

Avalanche: A large mass of material falling or sliding rapidly due to the force of gravity. In many cases, water acts as a catalyst and/or lubricant. Avalanches often are classified by what is moving, such as a snow, ice, soil, or rock avalanche. A mixture of these materials is commonly called a debris flow.

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Basalt: Volcanic rock (or magma) that is generally dark in color, contains 45 to 54 percent silica, and is rich in iron and magnesium. An eruption of basaltic magma is generally quiet, and results in flows (both vesicular and non-vesicular) and breccias. Undersea eruptions commonly result in the formation of "pillow lavas." Basalt represents the initial differentiated material erupted by the earth at spreading centers, and is considered by GeoMan to be the "blood of the earth."

Bedrock A general term for any consolidated rock.

Bentonite A clay material composed principally of the mineral montmorillonite. It has a great affinity for fresh water and when hydrated will increase its volume more than seven times. Water/bentoninte suspensions are essentially impermeable. Commonly used as a sealant for ponds.

Biostratigraphy: The study and classification of rocks and their history based on their fossil content.

Block: Angular chunk of solid rock ejected during a volcanic eruption.

Bomb: Fragments of molten or semi-molten rock, several inches to several feet in diameter, which are blown out during an explosive volcanic eruption. Because of their semi-plastic condition, bombs are often modified in shape during their flight or upon impact.

Breccia: Angular fragments of material, commonly formed by physical weathering processes or explosive volcanic activity.

Brittle-Ductile Transition Zone: The location at depth within the earth's crust where the temperature and pressure have risen to such a high level that directed stress results in plastic deformation as opposed to fracturing and faulting.

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Caldera: The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades.

Calorie: A unit of heat energy. The amount of heat required to raise the temperature of 1 gram (cubic Centimeter) of water 1 degree Celsius. Also, the substance which gives food its flavor.

Chattermarks: Erosional features associated with alpine glaciers.

Cinder cone: A volcanic cone built entirely of loose fragmented material (pyroclastics.)

Collection: The accumulation of precipitation into surface and underground areas, including lakes, rivers, and aquifers.

Comet: An object which circles the sun in a non-circular orbit. Commonly made up of a large mass of rock debris and ice. Basically, a giant snockball from space.

Composite volcano: A steep volcanic cone built by both lava flows and pyroclastic eruptions.

Condensation: The change of state of water from the vapor to the liquid phase. Results in liberation of 80 calories per cubic centimeter.

Connate water: Water included in the groundwater which is derrived from the rock itself, as opposed to water which has percolated down from the surface.

Continental crust: Solid, outer layers of the earth, including the rocks of the continents.

Continental drift: The theory that horizontal movement of the earth's surface causes slow, relative movements of the continents toward or away from one another.

Continental shelf: Portions of the continental land masses covered by sea water. Extend varying distances outward from the exposed continental margins. Usually, the continental shelf will be wider along a passive continental margin, and narrower along an active margin.

Crater: A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent.

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Dacite: Volcanic rock (or lava) that characteristically is light in color and contains 62 to 69 percent silica and moderate amounts of sodium and potassium.

Debris avalanche: A flow of unsorted masses of rock and other material downslope under the influence of gravity. Water is commonly involved as a catalyst and/or lubricant. For example: a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacial ice, trees and other debris, and hot pyroclastic material was associated with the May 18, 1980 eruption of Mt. St. Helens. Most of the deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche resulting from the eruption.

Density: A measure of how tightly packed the atoms of a substance are. Measured in grams per cubic centimeter. Varies by the mineral or substance. For example, gold has a high density, while quartz has a low density. See also "specific gravity."

Detachment plane: The surface along which a landslide disconnects from its original position.

Dew point: The temperature (elevation) where adiabatic cooling results in the initiation of condensation of water vapor into cloud droplets.

Di-polar: The arrangement of the hydrogen atoms of a water molecule at 105 deg. across the oxygen results in a slight electrical charge to the molecule. It also results in water molecules looking like Mickey Mouse instead of Alfred E. Newman.

Dome: A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent, often circular in plane view and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome.

Dormant volcano: This term is used to describe a volcano which is presently inactive but which may erupt again. The major volcanic cones of the Cascade Mountains (in Washington, Oregon, and California) are believed to be dormant rather than extinct.

Drift (glacial): General term for material deposited by a glacier.

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Ejecta: Material that is thrown out by a volcano, including pyroclastic material (tephra) and, from some volcanoes, lava bombs.

Erosion: The movement of weathered material downslope under the influence of gravity. Water acts as a catalyst and as a lubricant. Some common types of erosion includes landslides, rockfalls, creep, etc. Erosion takes weathered material and puts it in a river so it can be transported to the beach (see Strickler's 3rd Law of GeoFantasy).

Erratic (glacial): Large rocks or other debris deposited by a glacier, usually in an area far removed from its source. Commonly used to indicate a big chunk of debris which is clearly out of place and shouldn't even be where it is.

Eruption: The process by which solid, liquid, and gaseous materials are ejected into the earth's atmosphere and onto the earth's surface by volcanic activity. Eruptions range from the quiet overflow of liquid rock to the tremendously violent expulsion of pyroclastics.

Eruption cloud: The column of gases, ash, and larger rock fragments rising from a crater or other vent. If it is of sufficient volume and velocity, this gaseous column may reach many miles into the stratosphere, where high winds will carry it long distances.

Eruptive vent: The opening through which volcanic material is emitted.

Evaporation: The change of state of water from the liquid to vapor phase. Requires the addition of 80 calories per cubic centimeter.

Evapotranspiration: Water used by plants and animals and subsequently returned directly to the atmosphere.

Evolution: The theory that living organisms mutate and change, generally from simple to increasingly complex forms.

Extinct volcano: A volcano that is not presently erupting and is not likely to do so for a very long time in the future.

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Fault: A crack or fracture in the earth's surface in which there has been movement of one or both sides relative to the other. Movement along the fault can cause earthquakes or, in the process of mountain-building, can release underlying magma and permit it to rise to the surface as a volcanic eruption.

Firn: The intermediate "granular" stage which occurs during the conversion of snow to glacial ice.

Fissures: Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.

Flank eruption: An eruption from the side of a volcano (in contrast to a summit eruption.)

Fossil: Evidence of past life on earth. Can include the preserved hard and soft parts of plants and animals, tracks and burrows, whole organisms preserved intact in amber or tar, and fossilized dung. ANY evidence of life constitutes a fossil.

Floodplain: The low relief lands bordering a stream or river, common to the mature and old age stages of stream development. Floodplains store excess water in times of high water, and excess sediments in times of low water. Beware of building your dream house on a floodplain - they tend to get rather wet at irregular intervals.

Fumarole: A vent or opening through which issue steam, hydrogen sulfide, or other gases. The craters of many dormant volcanoes contain active fumaroles.

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GeoMan: Your's truly, and a real GeoGeek.

Geothermal energy: Energy derived from the internal heat of the earth.

Geothermal power: Power generated by using the heat energy of the earth.

Glacial ice: Naturally occurring ice which exhibits internal plastic flow and deformation.

Glacial abrasion: A copmmon mechanical weathering process where rock and debris frozen into the sides and bottom of a glacier act like sandpaper and wear down the bedrock the glacier is mocing across.

Glacial quarrying (plucking): A common mechanical weathering process in alpine glaciated terrain where glacial ice frozen into cracks in the bedrock literally "pluck" rock material from the valley floor.

Glacial polish: Polished bedrock surfaces left behind after melting of glacial ice. The polishing is probably due to very fine grained rock flour carried at the base of the ice.

Graben: An elongate crustal block that is relatively depressed (downdropped) between two fault systems.

Groundwater: Water stored beneath the surface in open pore spaces and fractures in rock.

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Harmonic tremor: A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault.

Heat transfer: Movement of heat from one place to another.

Horizontal blast: An explosive eruption in which the resultant cloud of hot ash and other material moves laterally rather than upward.

Hot-spot volcanoes: volcanoes related to a persistent heat source in the mantle.

Hydrologic cycle: The transfer of water between numerous temporary storage reservoirs. These include the ocean, rivers and streams, glacial ice, beer cans, dogs and cats, groundwater, and the atmosphere.

Hydrothermal reservoir: An underground zone of porous rock containing hot water.

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Isostasy: The vertical readjustment of the surface of the earth due to the addition or removal of weight. Commonly associated with the advance and retreat of glacial ice.

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Kinetic energy: The energy of motion.



Lahar: A torrential flow of water-saturated volcanic debris down the slope of a volcano in response to gravity. A type of mudflow.

Langley: The unit of solar energy relating to the amount which reaches a specific area of the earth's surface. In general, more "langleys" reach the surface of the earth at the equator than at the poles.

Lapilli: Literally, "little stones;" round to angular rock fragments measuring 1/10 inch to 2-1/2 inches in diameter, which may be ejected in either a solid or molten state.

Lava: Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified igneous rock.

Lava Flow: An outpouring of lava onto the surface from a vent or fissure. Also, a solidified tongue-like or sheet-like body formed by outpouring lava.

Lava tube: A tunnel formed when the surface of a mafic lava flow cools and solidifies, while the still-molten interior flows through and drains away. These can insulate the flow and allow it to travel great distances.

Leeward: The side facing away from the wind. When speaking of a mountain range, these areas are generally hotter and drier than on the windward side.

Loess: Very fine-grained sediments deposited by wind action. Commonly associated with the margins of continental ice sheets. Large expanses of loess from the recent ice age are in large part responsible for the bountiful corn and wheat fields of the American Midwest.

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Magma: Molten rock beneath the surface of the earth.

Magma chamber: The subterranean cavity containing magma. When a conduit is opened to the surface, a volcanic eruption is possible.

Mantle: The zone of the earth below the crust and above the core.

Magnitude: A numerical expression of the amount of energy released by an earthquake, determined by measuring earthquake waves on standardized recording instruments (seismographs.) The number scale for magnitude is a modified logarithmic value, rather than arithmetic, and the numbers get real big, real fast; a magnitude 9 earthquake, for example, is 33 times greater than a magnitude 8 earthquake, 1089 times greater than a magnitude 7 earthquake, 35937 times greater than a magnitude 6 earthquake, and so on. The short version? Small quakes don't really do much to relieve stress in the crust.

Marker horizon (or bed): A distinctive horizon which is used for regional correlation of lithology. A good marker horizon is distinctive, widespread, and represents a relatively short period of geologic time. For example, ash from a volcanic eruption, debris from a meteorite impact, etc. It is GeoMan's opinion that humans will represent one of the earth's finest marker horizons in the geologic record of the future. Our effect on the surface is certainly distinctive and widespread, and, at the rate we are going, it is likely that our species will have a relatively short lifespan (speaking in terms of geologic time, of course).

Metamorphic: From the Greek "meta" (change) and "morph" (form). Commonly occurs to rocks which are subjected to increased heat and/or pressure. Also applies to the conversion of snow into glacial ice.

Mineral: A naturally occurring, inorganic, crystalline solid with a definite internal structure and chemical composition.

Moraine: General term for material deposited beneath, along the sides, and/or at the terminus of a glacier. Also, what we get here in Oregon during the fall, winter, and spring. See also till.

Mudflow: A flowage of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar.

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Norite: An igneous rock with mafic composition.

Nuée ardente: A French term applied to a highly heated mass of gas-charge ash which is expelled with explosive force down the mountainside. Common to intermediate volcanoes. Can be quite deadly. Also known as a "glowing avalanche."

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Obsidian: A black or dark-colored volcanic glass, usually of rhyolitic (felsic) composition.

Oceanic crust: The earth's crust where it underlies oceans.

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Pahoehoe: Hawaiian term for a fluid volcanic eruption resulting in broad basaltic shield volcanoes. The highly fluid magma flows readily, and hardens into ropey forms as it cools. It can be very impressive to view an active flow!

Phreatic eruption (explosion): An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.

Pillow lava: Interconnected, sack-like bodies of lava that form underwater.

Plastic deformation (or flow): Permanent bending or folding of rock (or ice) as a result of directed pressure. In rock, usually occurs below the Brittle-Ductile Transition Zone, and is commonly associated with metamorphism.

Plate tectonics: The theory that the earth's crust is broken into about fragments (plates,) which move in relation to one another, shifting continents, forming new ocean crust, and causing volcanic eruptions.

Plug: Solidified lava that fills the conduit of a volcano. Plugs (also called volcanic necks) are usually more resistant to erosion than the material making up the surrounding cone, and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.

Plug dome: The steep-sided, rounded mound formed when viscous lava wells up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged.

Pluton: A large igneous intrusion formed at great depth in the crust.

Potential energy (gravitational): The stored energy of a substance. Water has a lot of this if there is an elevation difference. Potential energy can be converted to kinetic energy if the water (or other substance) is allowed to move.

Precipitation: Any condensed water falling from the atmosphere to the surface of the earth. Common types include rain, snow, sleet, and hail.

Problem: A situation which is generally uncomfortable, or otherwise undesirable. I always seem to have several - how about you?

Pumice: Light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles.

Pyroclastic: Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.

Pyroclastic flow: Lateral flowage of a turbulent mixture of hot gases (≈400°C) and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (100 miles an hour or more). Also known as a "glowing avalanche" or "nueé ardente."

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Rhyolite: Volcanic rock (or lava) that characteristically is light in color, contains 69 percent silica or more, and is rich in potassium and sodium. It is fine grained, which although different in texture, has the same composition as granite

Ridge, Oceanic: A major submarine mountain range. Commonly the sites of crustal rifting and plate separation, and the eruption of mafic basaltic lavas.

Rift system: The oceanic ridges formed where tectonic plates are separating and a new crust is being created; also, their on-land counterparts like the East African Rift.

Ring of Fire: The regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean.

Rock flour: Finely ground rock material, usually associated with glaciers (or faults). Can be mixed with water and formed into loaves which, when baked for 45 minutes at 350°, are totally unedible.

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Seafloor spreading: The mechanism by which new seafloor crust is created at oceanic ridges and slowly spreads away as tectonic plates separate.

Sea level: The top of the ocean, where the water meets the atmosphere. It's not necessarily level!

Sediment: Rock debris commonly produced by mechanical or chemical weathering processes.

Seismograph: An instrument that records seismic waves; that is, vibrations of the earth. Used to record and measure earthquakes.

Shearing: The motion of surfaces sliding past one another.

Shield volcano: A gently sloping volcano in the shape of a flattened dome, built almost exclusively of mafic lava flows. The Hawaiian Islands are a good example.

Silica: A chemical combination of silicon and oxygen.

Sniceball: A snowball which has been stored in the freezer for several months (or more). Useful for surprising unwelcome visitors during the spring and summer months.

Snirtball: A combination of snow and dirt. Snirtballs are produced by accident when the total snowfall on bare ground is less than 0.537 inches.

Snockball: A combination of snow and rock: generally an innocent-looking snowball with a dense, rocky core. Snockballs are always premeditated, and are not known to occur naturally on earth. Giant snockballs from space (also called comets) may be responsible to the initial introduction of water onto our planet.

Snowball: A spherical accumulation of water in the crystalline form.

Snowline: The lower limit of any year's permanent snowfall. Separates the Zone of Accumulation from the Zone of Ablation.

Solid state: In metamorphism, indicates the change of mineral identity without melting. All ion migration occurs while the rock (or pre-glacial ice) is still solid.

Specific gravity: A measure of how tightly packed the atoms of a substance are. Varies by the mineral or substance. Example, gold has a high specific gravity, while quartz has a low specific gravity. See also "density."

Spines: Horn-like projections formed upon a lava dome.

Spring: The time between winter and summer.

Spring: A surface flow of groundwater which occurs any time the water table intersects the surface.

Stratovolcano: A volcano composed of both lava flows and pyroclastic material. Also called "Composite" volcanoes. Common at convergent boundaries. Excellent examples in the U.S. include Mt. St. Helens, Mt. Shasta, and the other peaks of the Cascade Range of California, Oregon, and Washington.

Striations (glacial): Grooves eroded into bedrock by rock debris frozen into the base of a glacier.

Strike-slip fault: A nearly vertical fault with side-slipping displacement.

Subduction zone: The zone of convergence of two tectonic plates, one of which usually overrides the other.

Sublimation: The direct change from the solid to the vapor phase (without passing through the liquid phase). Commonly occurs in ice and snow fields on sunny days above the snowline.

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Tephra: Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.

Till (glacial): General term for material deposited by a glacier. See also moraine.

Tsunami: A great sea wave produced by a submarine earthquake, volcanic eruption, or large landslide. Commonly (but erroneously) called a "tidal wave," tsunamis can cause great damage due to flooding of low coastal areas.

Tuff: Rock formed of pyroclastic material.

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Vapor (water): Water in the gaseous state.

Vent: The opening at the earth's surface through which volcanic materials reach the surface.

Vesicular basalt: Holes and other openings in basaltic flow which are the result of trapped gas bubbles. Vesicles are often filled at a later date with a wide variety of materials, including, quartz, agate, zeolites, and many other minerals.

Viscosity: A measure of resistance to flow in a liquid (molasses in January has high viscosity while molasses in August has lower viscosity).

Volcanic Neck: Solidified lava that fills the conduit of a volcano. Volcanic necks (also called plugs) are usually more resistant to erosion than the material making up the surrounding cone, and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.

Vulcan: Roman God of fire and the forge, after whom volcanoes are named.

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Water cycle: The transfer of water between numerous temporary storage reservoirs. These include the ocean, rivers and streams, glacial ice, beer cans, dogs and cats, groundwater, and the atmosphere.

Weathering (surface): "Making little ones out of big ones." Waethering includes the processes which mechanically and chemically break down the mountains into little pieces, so they can be eroded and transported to the beach (see Strickler's 3rd Law of GeoFantasy).

Well: A hole dug into the ground in the attempt to intersect water or other subsurface fluids.

Windward: The side facing into the wind. When speaking of a mountain range, these areas are generally cooler and wetter than on the leeward side.

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Zone of ablation (or wastage): The area below the snowline where snow melt exceeds snowfall, and material is lost from a glacier.
Zone of accumulation: The area above the snowline where snowfall exceeds snow melt, and material is added to a glacier.

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